This Department of Energy funded study represents the first significant investigation of polyimide flex as a substrate material for high temperature (>200°C) electronics packaging for downhole oil and gas exploration. This program examined at three key areas of package development: conductor adhesion to flex, through hole via reliability in flex, and high temperature interconnect methods for passive devices on flex. High temperature storage testing of different adhesion layers to flex was performed at 200 and 250°C in air and nitrogen for as long as 1000 hrs. Multiple adhesion materials and thicknesses were evaluated by measuring the peel strength of copper traces to polyimide. A Cr adhesion layer was selected due to its ability to maintain high peel strength during the high temperature storage testing. Thermal cycling of through hole vias between room temperature and 250°C for greater than 1200 cycles showed little degradation. Thermal cycling was also performed on passive components attached to flex using four high temperature solders. Results up to 277 cycles is reported.